Show summary and description; always Markdown

As a new gem author, I was pretty puzzled by the contrast between:

* The specification docs, which say: "The description should be more detailed than the summary." ( http://guides.rubygems.org/specification-reference/#summary 
* The actual output on the site, which only shows the summary if both are present.

Additionally, it seems like many people try to put line breaks in these fields, but the site just puts them all in a `<p>` unless the text happens to match the very specific regex: `/^==/`

So this patch should rectify both. I hope you like it, but if not, let's talk about the options. Thanks!